A FULLY MAINS OPERATED SOLID STATE POWER DELAY CIRCUIT

 

V.Vidyalal, V. Sivanand and K. Rajasree,

Floyds Inc., "KAILAS"

EDAKKAD P.O, CALICUT,

INDIA - 673005

Or

V.Vidyalal

Uni-Tel. Inc, 8111 LBJ Freeway, Suite 1100

Dallas, Texas-75251,

USA

lal@uni-tel.com

 

 

Introduction

This circuit can be used to turn off appliances, porch lights, or any other load after a certain time interval. The unique features of this circuit are 1) Light Weight 2) Fully mains operated 3) Uses fully solid state components (Hence no maintenance required in the long run) 4) Inexpensive 5) Low stand by current and 6) Silent operation because of solid state switching.

For driving heavier loads all one need is to substitute an appropriate triac leaving the rest of the circuit unaltered.

 

THE CIRCUIT

 

 

The circuit can be basically divided into four major parts :-

 

  1. THE DELAY CIRCUIT: Built around the popular CMOS 4049 IC, this circuit can provide delays up to a couple of minutes. A Schmitt trigger is formed using two gates from the HEX-inverting package. When switch S2 is pressed, C8 is charged. This biases the transistor T1 into conduction, causing the load to be turned "ON". After a certain time interval as set by R6 the voltage across C8 decays until the gate can no longer "hold" and it changes state by shutting off the thyristor and thus the load is de-energized. The time interval depends on R5, R6 and C8. For R5 + R6 = 1M , and C8 = 47 m F a time interval up to two minutes is possible, which is ideal for almost all applications. Use low leakage tantalum capacitor for C8.

    2.  

  2. THE THYRISTOR DRIVE CIRCUIT : T1 is a discrete emitter follower output stage which provides the relatively high trigger current required by the triac. The LED1 in the emitter glows ( Busy Indicator ) when the transistor is biased into conduction by the gates of IC2. The "Snubber circuit" consisting of R2 and C2 across the triac filters out the RF interference caused by its rapid switching.

    3.  

  3. THE CIRCUIT " ON " FLASHING INDICATOR DRIVE CIRCUIT: This is built around the rest of the four inverters in IC2. Two inverters are configured as an oscillator with supporting components R10, R11 and C9. Varying R11 can control the flashing of the LED2. The output of the oscillator is connected to two inverters connected in parallel, which is used, as a buffer to supply the appropriate current required by the LED2.

    4.  

  4. THE MAINS PSU CIRCUIT: It comprises of R1, D1, D1 and C3. It provides the required DC voltage for the circuit. C1 works as a non-dissipative element, its reactance at the mains frequency limits the current. Since the voltage and current across the capacitor are 90° out of phase, there is ideally no wastage of power. R1 is the initial surge limiter, diode D1 rectifies and D2 ensures proper polarity for C3. C4 removes high frequency transients riding on the power supply rails to some extend. IC1 is a 3-pin regulator, which provides the necessary voltage for the rest of the circuit. C5 smoothes the ripples, C6 the high frequency transients and prevents oscillations of IC2.

    5.  

    OPERATION: Testing is very simple. First switch on the power supply using S1. LED2 starts flashing, the rate depending on R11. Adjust R11 (with a plastic trimming tool) according to the user’s requirement. Press S2. LED1 should light up and when S2 is released, LED1 should remain lit for some time depending on the value of R6 (adjust R6 for the required time). When LED1 lights up, the triac is sure to fire and energize the load. After a short interval of time the load is disabled.

     

    PARTS LIST

    Semiconductors:

    IC1 - 7809, 3 Pin voltage regulator. IC2 - CD4049B CMOS TRIAC - According to load current requirement

    Q1 - BC 548B NPN Transistor D1, D2 - IN4007 Silicon Diodes

    RESISTORS:

    (ALL ¼ W, 5% Carbon, unless otherwise specified.)

    R1 = 100 , 5W R2 = 100 , 1W R3 = 1k2

    R4 = 330 R5 = 1M R6 = 1M Preset

    R7 = 10M R8 = 100 R9 = 1M

    R10 = 2M2 R11 = 1M Preset R12 = 680

    CAPACITORS:

    C1 = 2.2m F/400 V, Metallised Polyester C2 = 100nF/400 V, Polyester

    C3 = 1000m F/50V Electrolytic C4 = 100nF/50V, Ceramic

    C5 = 1000m F/50V Electrolytic. C6, C7 = 100nF/50V, Ceramic.

    C8 = 47m F/12V, Tantalum bead. C9 = 1m F/50v, Polyester.

    SWITCHES:

    S1= SPST SWITCH, S2 =PUSH BUTTON SWITCH

    MISCELLANEOUS:-

    Fuse = 1A, slow blow, LED1 = Green, LED2 = Red, flexible wires, mains cord, mains socket, LED holders etc.

     

    CAUTION: Since the whole circuit is connected to the live mains, one should be extremely careful. It is advisable to enclose the whole unit inside a plastic cabinet, leaving provisions for input and output cables.

    *-*-*-*-*-*-*-*-*

     NOTES

  5. WE the authors are aware that many improvements and refinements can be made, but the idea is to keep it simple as possible, and executed by anyone who can make a circuit board and has the patience !!!!.

WARNING AND DISCLAIMER (!!!!!!!)

Mains-powered and also equipment like the one described above contains high AC voltages. These can be potentially lethal. If you do not know how to safely handle the inside of equipment containing high voltages, please do not attempt to open the equipment. Before carrying out any work inside equipment containing high Voltages, ensure that you disconnect the Unit from the Mains. (2) The information included herein is provided as-is, with no warranties expressed or implied. No responsibility on the part of the author is assumed for the technical accuracy of the information given herein or the use or mis-use of said information.

The equipment described in this article was designed, fabricated, and tested spending my own personal time and using my own personal resources.

 

 The information on this site is Copyright © 1998 of Dr.LAL. You may link to these pages or print them out for your own personal use, but no parts of these pages can be reproduced, sold, or transmitted in any form without my explicit written permission. By copying or paraphrasing the intellectual property on this page, you're automatically signing a binding contract and agreeing to be billed $1000 payable immediately. © Dr LAL's Homepage (CKT.DESIGN) 1998.

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